In the VirtualBox user manual, there is a little command refference guide at the beginning of the section 8, VBoxManage refference. For the audio options related to the "modifyvm" subcommand, it states:

[-audio none|null|dsound|solaudio|oss|coreaudio]

I haven't found explanations on how *all* these options work, neither at section 8.5.4, nor at section 3.7.4 of the user manual. I guess that "none" means no sound at all, and "oss" lelates to the Linux Open Sound System. How about the others?

Besides it, I'd like to know whether ac97 and sb16 are the only audio controlers supported for now, or the are other undocumented ones.

"null" means the guest is told there is a sound card, but behind the scenes VirtualBox will discard all sound output. The guest thinks it is making sound, but the person sitting at the computer will never hear anything.

I don't know the other ones. Nor have I heard of anyone talking about any undocumented sound card emulations, other than SB16 and ICH AC97.

The options tell VirtualBox how to export sound to the host. As such it is NOT related to the soundcard VirtualBox presents to the Guest. For the latter the choice is pretty much as you describe it: AC97 or SB16 or - NONE